1. Field of the Invention
The present invention concerns a method for operating a magnetic resonance apparatus (MR) with a steady-state pulse sequence. Moreover, the invention also concerns a magnetic resonance apparatus with a sequence for operating an RF unit and a gradient system for implementing such a method,
2. Description of the Prior Art
Fast imaging methods enable the implementation of magnetic resonance examinations in significantly shortened time spans with a significantly reduced repetition time between successive acquisition cycles. They are based on adopting a dynamic equilibrium of the (longitudinal and transverse) magnetization of an examination subject with the aid of steady-state pulse sequences, for example using gradient echoes. After each RF pulse, the magnetization again assumes the same value, meaning that a signal known as a steady-state signal (equilibrium signal) is generated after a specific transient effect duration. The mechanism of the equilibrium formation is explained, for example, in chapter 6.1.8 in “Bildgebende Systeme für medizinlsche Diagnostik”, H. Morneburg, (1995) Publicis MCD Verlag.
The equilibrium of the magnetization is achieved after a series of RF pulses with a pulse interval of the repetition time TR, which deflect the magnetization of the spin system by an angle α from its alignment along a basic magnetic field (z-direction). The repetition time TR does not allow a complete relaxation of the magnetization. Small deflection angles effect a relatively large transverse magnetization, which leads to a correspondingly high signal intensity.
A wide variety of imaging methods have been realized that use different steady-state pulse sequences, for example Fast Low Angle Shot (FLASH), Fast Imaging with Steady State Procession (FISP) and modifications of these sequences, PSIF and True-FISP. The pulse sequences differ, for example, in the number of the spatial components of the magnetization which is located in the equilibrium. For example, in the True-FISP pulse sequence a transverse magnetization MXY is refocused and brought into the equilibrium state by means of a phase coder gradient, a readout gradient and a slice selection gradient. Furthermore, the various RF pulse sequences differ, for example, in their transient effect behavior in the dynamic equilibrium state. An overview of the various RF pulse sequences on the basis of a dynamic equilibrium magnetization is given in “Imaging Sequences in Magnetic Resonance Tomography and their Clinical Application”, electromedica 64 (1996) no. 1, page 23–29 (part 1), electromedica 64 (1996) no. 2, page 48–51 (part 2), electromedica 65 (1997) no. 1, page 8–14 (part 3). The True-FISP pulse sequence is described in detail in U.S. Pat. No. 4,769,603.
Fast imaging methods generally exhibit the disadvantage of image artifacts possibly arising due to a static magnetic field inhomogeneity or due to an existing chemical shift. A further disadvantage is a susceptibility to contributions from a magnetization that is generated in the environment of the slice to be measured. These can additionally arrive in the slice to be measured due to effects known as inflow effects, for example, in which magnetization generated outside of the slice of interest flows into the slice of interest during the imaging.
The methods described below are based on simple MZ preparation pulses that allow the blood to appear dark.
J. F. Glockner et al. compare a FIESTA sequence with a Double Inversion Fast Spin-Echo sequence in “Cardiac Imaging with Single Shot Black Blood FIESTA: A comparison with Double Inversion Fast Spin-Echo Imaging”, ISMRM 2002 (Hawaii), The Black Blood FIESTA sequence thereby uses a non-selective RF inversion pulse which is followed by a slice-selective inversion pulse.
K. S. Nayak et al. specify a real-time method for black blood depiction with the aid of spatial saturation pulses in “Real Time Black Blood MRI Using Spatial Presaturation”, JMRI 13, p. 807 (2001).
In “High Resolution 3D Fast Spin-Echo Black Blood Coronary MRA”, ISMRM 2001 (Glasgow), p. 170, M. Stuber et al, show that the expansion of what is known as the “Black Blood Coronary” imaging with a 3D acquisition technique can effect further improvements in the signal-to-noise ratio. This would lead to an improved spatial resolution. A combination of a “3D-Fast-Spin-Echo method” with a double inversion pulse is proposed for this.
In “Initial Experiences with Coronary Vessel Wall Imaging on a 3T Whole Body System”, ISMRM 2002 (Hawaii), R. M. Botnar et al. indicates that the coronal “Vessel Wall” representation assumes a high spatial resolution due to the small size.